1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing an electrode for a battery employing a metal porous body having three-dimensionally continuous pores as a substrate, to be applied to an alkaline storage battery or the like.
2. Description of the Prior Art
As a method of manufacturing an electrode which is applied to an enclosed-cell type alkaline storage battery etc., a non-sintering type manufacturing method of filling a metal porous body having three-dimensionally continuous pores with active material powder has been studied to take the place of a general sintering type manufacturing method. This non-sintering type manufacturing method is considered as being effective for reducing the cost and improving performance since manufacturing steps thereof are simplified and the time required for manufacturing can be reduced, while energy density of the electrode can be easily increased.
A concrete example of this manufacturing method comprises the following steps: A felt-type sintered body of nickel fiber, called a nickel mat, having physical properties of 20 .mu.m in fiber diameter, 50 .mu.m in average pore diameter and 93% in porosity is employed as a substrate, to be filled with a slurry prepared by mixing active material powder mainly composed of nickel hydroxide powder of 20 .mu.m in average particle size into a binder solution. Then the sintered body is dried and pressurized/compressed to provide an electrode.
However, the nickel mat has originally been developed as a filter, and hence the active material powder is filtrated by the surface of the nickel mat, which allows passage of only the liquid, in operation for filling of the active material slurry. Thus, the active material cannot be filled into the nickel mat in prescribed volume.
In Japanese Patent Publication Gazette Nos. 31832/1984, 24492/1984 or 37665/1981, a slurry supplied by a rubbing member which is transferred to the surface of a substrate and is mechanically rubbed into the substrate in order to solve the aforementioned problem. In such a method, however, the apparatus is complicated in structure and it is difficult to homogeneously fill the substrate with the slurry.
Japanese Patent Laid-Open Gazette Nos. 81868/1984 or 143270/1984 discloses a method of spraying a slurry of an active material to a substrate with air, to fill the same by pressure. In this method, however, the substrate must be sufficiently rigid to withstand high pressure spray. Further, the filling volume is reduced since the slurry is supplied with air, while it is difficult to fill the slurry in constant volume.
In the general sintering type manufacturing method, a substrate is impregnated with a solution of raw salt containing an active material, and then the raw salt is converted into the active material by a chemical, electrochemical or thermal method. In this method, the filling operation is repeated several times since the active material cannot be filled in the prescribed volume at once, due to large differences in density between the active material and the salt solution. Thus, every filling volume can be measured so that the total filling volume accurately reaches a prescribed value.
In the non-sintering type manufacturing method, on the other hand, it is difficult to obtain the filling volume to a prescribed value since the filling operation is performed only once. As an improved method of filling a metal porous body having three-dimensionally continuous pores with an active material, Japanese Patent Laid-Open Gazette No. 10833/1978 discloses a method of homogeneously filling the active material by sufficiently pre-moistening the interior of the metal porous body with a liquid. Further, Japanese Patent Laid-Open Gazette No. 10834/1978 discloses a method of filling paste of an active material having a high moisture content in the first place and then filling paste of an active material having a low moisture content. However, these methods fail to provide an improvement for attaining filling volume to a prescribed value.
Japanese Patent Laid-Open Gazette No. 105363/1987 discloses a method of filling a metal porous body with active material paste of 1000 to 4000 cp. in viscosity by rotating a roller having a plurality of grooves. In the method of this prior art, however, the roller is submerged in the paste, and hence air contained in the metal porous body must be outwardly extruded into the paste. Since it is difficult to smoothly expel such air from the porous body in a direction opposite to that for filling the paste, air cannot be sufficiently replaced by the paste in the porous body. Thus, it is difficult to uniformalize the filling volume of the paste in the porous body. Japanese Patent Laid-Open Gazette No. 52863/1988, which discloses similar technique, has the same problem.
Japanese Patent Laid-Open Gazette No. 40837/1978 describes a method of filling a metal porous body having three-dimensionally continuous pores with a paste-type substance mainly composed of active material powder, pressure-molding the same and then impregnating the pressure-molded body with a dispersion medium of fluororesin. According to this method, the active material can be prevented from falling caused by repeated charge/discharge by increasing the binding power of the active material with respect to the metal porous body. In this case, however, a new problem is created; specifically the active material inevitably falls from the substrate in impregnation with the dispersion medium. This is because the active material which is weakly held on the substrate surface falls off with the fluororesin dispersion medium when the substrate is lifted from the dispersion medium, since the substrate is impregnated with fluororesin being in the form of a liquid. In the method of impregnating the pressure-molded body with the fluororesin dispersion medium, further, the amount of adhesion of fluororesin is dispersed if the substrate surface is inhomogeneous in a former stage. Although there has been proposed a method of adding a binder of fluororesin or the like into the paste of an active material, internal resistance of the electrode is increased notwithstanding improvement in binding property.